Miniaturisation of an antenna consists in providing antenna dimensions of less than around one sixth of the wavelength, and the efficiency of the antenna is reduced due to the fact of these small dimensions. In fact, a dipole antenna optimised from the point of view of efficiency should have dimensions in the order of the half-wavelength, i.e., for example, 15 cm for 500 MHz. A miniaturised antenna would instead have a length of 5 centimetres in its largest dimension, more suitable for a portable communication device that must be capable of being handheld.
Problems encountered in antenna miniaturisation include interactions between the antenna and its immediate environment, and one object of the invention is to provide an antenna geometry that minimises these interactions, which would be detrimental to the efficiency of the antenna.
Meander antennas have already been proposed in which the antenna is formed by a conducting wire folded over itself in order to retain a sufficient total wire length (close to one quarter of the wavelength), while restricting the overall size.
The 1 shows the principle of a monopole meander antenna, made up of a wire F mounted above a ground plane M and folded over itself. The height above the ground plane is around three times less than the total length of the unfolded wire.
FIG. 2 shows a different configuration in which the directions of elongation of the antenna wire are parallel and not perpendicular to the ground plane, and in which the wire is folded multiple times. In the example shown, there are ten elbows of folding in the area of which the direction of the wire is reversed. The height above the ground plane is, for example, five to ten times less than the total length of the unfolded wire.
Antenna structures formed by etching of printed circuit boards have also been proposed. The conducting wires of the antenna and the ground plane are etched onto the surface of the board. The conducting wires can be etched on one surface of the board and the ground plane on a different surface of the board. The height is particularly reduced since it is limited to the thickness of the board and the conducting layers deposited on the board. FIG. 3 shows an example of this type of antenna; the left part of the figure shows one surface of the board, and the right part shows the opposite surface. The ground plane M is etched on one surface. An antenna wire F is etched onto a different surface.
FIG. 4 shows a different form of compact antenna etched onto a printed circuit board, in which the antenna wire is folded in a spiral. The ground plane, not shown, is located on a different surface of the board.
Finally, slot antennas have been proposed in the prior art, in which the electromagnetic radiation is generated in an open, elongated slot in a flat conducting structure etched onto one surface of a printed circuit, the other surface of which forms a ground plane. The wider the slot, the lower the operating frequency can be.
However, the miniaturised antenna structures proposed to date have reduced radiation efficiency, i.e. a low ratio of the received electric power (which is the power of the source for a suitable antenna) to the radiated power, when the antenna is placed in an unfavourable environment.